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Collections in Java and Kotlin

Collections are groups of a variable number of items (possibly zero) that are significant to the problem being solved and are commonly operated on. This guide explains and compares collection concepts and operations in Java and Kotlin. It will help you migrate from Java to Kotlin and write your code in the authentically Kotlin way.

The first part of this guide contains a quick glossary of operations on the same collections in Java and Kotlin. It is divided into operations that are the same and operations that exist only in Kotlin. The second part of the guide, starting from Mutability, explains some of the differences by looking at specific cases.

For an introduction to collections, see the Collections overview or watch this video by Sebastian Aigner, Kotlin Developer Advocate.

Operations that are the same in Java and Kotlin

In Kotlin, there are many operations on collections that look exactly the same as their counterparts in Java.

Operations on lists, sets, queues, and deques

Description

Common operations

More Kotlin alternatives

Add an element or elements

add(), addAll()

Use the plusAssign(+=) operator: collection += element, collection += anotherCollection.

Check whether a collection contains an element or elements

contains(), containsAll()

Use the in keyword to call contains() in the operator form: element in collection.

Check whether a collection is empty

isEmpty()

Use isNotEmpty() to check whether a collection is not empty.

Remove under a certain condition

removeIf()

Leave only selected elements

retainAll()

Remove all elements from a collection

clear()

Get a stream from a collection

stream()

Kotlin has its own way to process streams: sequences and methods like map() and filter().

Get an iterator from a collection

iterator()

Operations on maps

Description

Common operations

More Kotlin alternatives

Add an element or elements

put(), putAll(), putIfAbsent()

In Kotlin, the assignment map[key] = value behaves the same as put(key, value). Also, you may use the plusAssign(+=) operator: map += Pair(key, value) or map += anotherMap.

Replace an element or elements

put(), replace(), replaceAll()

Use the indexing operator map[key] = value instead of put() and replace().

Get an element

get()

Use the indexing operator to get an element: map[index].

Check whether a map contains an element or elements

containsKey(), containsValue()

Use the in keyword to call contains() in the operator form: element in map.

Check whether a map is empty

isEmpty()

Use isNotEmpty() to check whether a map is not empty.

Remove an element

remove(key), remove(key, value)

Use the minusAssign(-=) operator: map -= key.

Remove all elements from a map

clear()

Get a stream from a map

stream() on entries, keys, or values

Operations that exist only for lists

Description

Common operations

More Kotlin alternatives

Get an index of an element

indexOf()

Get the last index of an element

lastIndexOf()

Get an element

get()

Use the indexing operator to get an element: list[index].

Take a sublist

subList()

Replace an element or elements

set(), replaceAll()

Use the indexing operator instead of set(): list[index] = value.

Operations that differ a bit

Operations on any collection type

Description

Java

Kotlin

Get a collection's size

size()

count(), size

Get flat access to nested collection elements

collectionOfCollections.forEach(flatCollection::addAll) or collectionOfCollections.stream().flatMap().collect()

flatten() or flatMap()

Apply the given function to every element

stream().map().collect()

map()

Apply the provided operation to collection elements sequentially and return the accumulated result

stream().reduce()

reduce(), fold()

Group elements by a classifier and count them

stream().collect(Collectors.groupingBy(classifier, counting()))

eachCount()

Filter by a condition

stream().filter().collect()

filter()

Check whether collection elements satisfy a condition

stream().noneMatch(), stream().anyMatch(), stream().allMatch()

none(), any(), all()

Sort elements

stream().sorted().collect()

sorted()

Take the first N elements

stream().limit(N).collect()

take(N)

Take elements with a predicate

stream().takeWhile().collect()

takeWhile()

Skip the first N elements

stream().skip(N).collect()

drop(N)

Skip elements with a predicate

stream().dropWhile().collect()

dropWhile()

Build maps from collection elements and certain values associated with them

stream().collect(toMap(keyMapper, valueMapper))

associate()

To perform all of the operations listed above on maps, you first need to get an entrySet of a map.

Operations on lists

Description

Java

Kotlin

Sort a list into natural order

sort(null)

sort()

Sort a list into descending order

sort(comparator)

sortDescending()

Remove an element from a list

remove(index), remove(element)

removeAt(index), remove(element) or collection -= element

Fill all elements of a list with a certain value

Collections.fill()

fill()

Get unique elements from a list

stream().distinct().toList()

distinct()

Operations that don't exist in Java's standard library

If you want to take a deep dive into zip(), chunked(), windowed(), and some other operations, watch this video by Sebastian Aigner about advanced collection operations in Kotlin:

Mutability

In Java, there are mutable collections:

// Java // This list is mutable! public List<Customer> getCustomers() { … }

Partially mutable ones:

// Java List<String> numbers = Arrays.asList("one", "two", "three", "four"); numbers.add("five"); // Fails in runtime with `UnsupportedOperationException`

And immutable ones:

// Java List<String> numbers = new LinkedList<>(); // This list is immutable! List<String> immutableCollection = Collections.unmodifiableList(numbers); immutableCollection.add("five"); // Fails in runtime with `UnsupportedOperationException`

If you write the last two pieces of code in IntelliJ IDEA, the IDE will warn you that you're trying to modify an immutable object. This code will compile and fail in runtime with UnsupportedOperationException. You can't tell whether a collection is mutable by looking at its type.

Unlike in Java, in Kotlin you explicitly declare mutable or read-only collections depending on your needs. If you try to modify a read-only collection, the code won't compile:

// Kotlin val numbers = mutableListOf("one", "two", "three", "four") numbers.add("five") // This is OK val immutableNumbers = listOf("one", "two") //immutableNumbers.add("five") // Compilation error - Unresolved reference: add

Read more about immutability on the Kotlin coding conventions page.

Covariance

In Java, you can't pass a collection with a descendant type to a function that takes a collection of the ancestor type. For example, if Rectangle extends Shape, you can't pass a collection of Rectangle elements to a function that takes a collection of Shape elements. To make the code compilable, use the ? extends Shape type so the function can take collections with any inheritors of Shape:

// Java class Shape {} class Rectangle extends Shape {} public void doSthWithShapes(List<? extends Shape> shapes) { /* If using just List<Shape>, the code won't compile when calling this function with the List<Rectangle> as the argument as below */ } public void main() { var rectangles = List.of(new Rectangle(), new Rectangle()); doSthWithShapes(rectangles); }

In Kotlin, read-only collection types are covariant. This means that if a Rectangle class inherits from the Shape class, you can use the type List<Rectangle> anywhere the List<Shape> type is required. In other words, the collection types have the same subtyping relationship as the element types. Maps are covariant on the value type, but not on the key type. Mutable collections aren't covariant – this would lead to runtime failures.

// Kotlin open class Shape(val name: String) class Rectangle(private val rectangleName: String) : Shape(rectangleName) fun doSthWithShapes(shapes: List<Shape>) { println("The shapes are: ${shapes.joinToString { it.name }}") } fun main() { val rectangles = listOf(Rectangle("rhombus"), Rectangle("parallelepiped")) doSthWithShapes(rectangles) }

Read more about collection types here.

Ranges and progressions

In Kotlin, you can create intervals using ranges. For example, Version(1, 11)..Version(1, 30) includes all of the versions from 1.11 to 1.30. You can check that your version is in the range by using the in operator: Version(0, 9) in versionRange.

In Java, you need to manually check whether a Version fits both bounds:

// Java class Version implements Comparable<Version> { int major; int minor; Version(int major, int minor) { this.major = major; this.minor = minor; } @Override public int compareTo(Version o) { if (this.major != o.major) { return this.major - o.major; } return this.minor - o.minor; } } public void compareVersions() { var minVersion = new Version(1, 11); var maxVersion = new Version(1, 31); System.out.println( versionIsInRange(new Version(0, 9), minVersion, maxVersion)); System.out.println( versionIsInRange(new Version(1, 20), minVersion, maxVersion)); } public Boolean versionIsInRange(Version versionToCheck, Version minVersion, Version maxVersion) { return versionToCheck.compareTo(minVersion) >= 0 && versionToCheck.compareTo(maxVersion) <= 0; }

In Kotlin, you operate with a range as a whole object. You don't need to create two variables and compare a Version with them:

// Kotlin class Version(val major: Int, val minor: Int): Comparable<Version> { override fun compareTo(other: Version): Int { if (this.major != other.major) { return this.major - other.major } return this.minor - other.minor } } fun main() { val versionRange = Version(1, 11)..Version(1, 30) println(Version(0, 9) in versionRange) println(Version(1, 20) in versionRange) }

As soon as you need to exclude one of the bounds, like to check whether a version is greater than or equal to (>=) the minimum version and less than (<) the maximum version, these inclusive ranges won't help.

Comparison by several criteria

In Java, to compare objects by several criteria, you may use the comparing() and thenComparingX() functions from the Comparator interface. For example, to compare people by their name and age:

class Person implements Comparable<Person> { String name; int age; public String getName() { return name; } public int getAge() { return age; } Person(String name, int age) { this.name = name; this.age = age; } @Override public String toString() { return this.name + " " + age; } } public void comparePersons() { var persons = List.of(new Person("Jack", 35), new Person("David", 30), new Person("Jack", 25)); System.out.println(persons.stream().sorted(Comparator .comparing(Person::getName) .thenComparingInt(Person::getAge)).collect(toList())); }

In Kotlin, you just enumerate which fields you want to compare:

data class Person( val name: String, val age: Int ) fun main() { val persons = listOf(Person("Jack", 35), Person("David", 30), Person("Jack", 25)) println(persons.sortedWith(compareBy(Person::name, Person::age))) }

Sequences

In Java, you can generate a sequence of numbers this way:

// Java int sum = IntStream.iterate(1, e -> e + 3) .limit(10).sum(); System.out.println(sum); // Prints 145

In Kotlin, use sequences. Multi-step processing of sequences is executed lazily when possible – actual computing happens only when the result of the whole processing chain is requested.

fun main() { //sampleStart // Kotlin val sum = generateSequence(1) { it + 3 }.take(10).sum() println(sum) // Prints 145 //sampleEnd }

Sequences may reduce the number of steps that are needed to perform some filtering operations. See the sequence processing example, which shows the difference between Iterable and Sequence.

Removal of elements from a list

In Java, the remove() function accepts an index of an element to remove.

When removing an integer element, use the Integer.valueOf() function as the argument for the remove() function:

// Java public void remove() { var numbers = new ArrayList<>(); numbers.add(1); numbers.add(2); numbers.add(3); numbers.add(1); numbers.remove(1); // This removes by index System.out.println(numbers); // [1, 3, 1] numbers.remove(Integer.valueOf(1)); System.out.println(numbers); // [3, 1] }

In Kotlin, there are two types of element removal: by index with removeAt() and by value with remove().

fun main() { //sampleStart // Kotlin val numbers = mutableListOf(1, 2, 3, 1) numbers.removeAt(0) println(numbers) // [2, 3, 1] numbers.remove(1) println(numbers) // [2, 3] //sampleEnd }

Traverse a map

In Java, you can traverse a map via forEach:

// Java numbers.forEach((k,v) -> System.out.println("Key = " + k + ", Value = " + v));

In Kotlin, use a for loop or a forEach, similar to Java's forEach, to traverse a map:

// Kotlin for ((k, v) in numbers) { println("Key = $k, Value = $v") } // Or numbers.forEach { (k, v) -> println("Key = $k, Value = $v") }

Get the first and the last items of a possibly empty collection

In Java, you can safely get the first and the last items by checking the size of the collection and using indices:

// Java var list = new ArrayList<>(); //... if (list.size() > 0) { System.out.println(list.get(0)); System.out.println(list.get(list.size() - 1)); }

You can also use the getFirst() and getLast() functions for Deque and its inheritors:

// Java var deque = new ArrayDeque<>(); //... if (deque.size() > 0) { System.out.println(deque.getFirst()); System.out.println(deque.getLast()); }

In Kotlin, there are the special functions firstOrNull() and lastOrNull(). Using the Elvis operator, you can perform further actions right away depending on the result of a function. For example, firstOrNull():

// Kotlin val emails = listOf<String>() // Might be empty val theOldestEmail = emails.firstOrNull() ?: "" val theFreshestEmail = emails.lastOrNull() ?: ""

Create a set from a list

In Java, to create a Set from a List, you can use the Set.copyOf function:

// Java public void listToSet() { var sourceList = List.of(1, 2, 3, 1); var copySet = Set.copyOf(sourceList); System.out.println(copySet); }

In Kotlin, use the function toSet():

fun main() { //sampleStart // Kotlin val sourceList = listOf(1, 2, 3, 1) val copySet = sourceList.toSet() println(copySet) //sampleEnd }

Group elements

In Java, you can group elements with the Collectors function groupingBy():

// Java public void analyzeLogs() { var requests = List.of( new Request("https://kotlinlang.org/docs/home.html", 200), new Request("https://kotlinlang.org/docs/home.html", 400), new Request("https://kotlinlang.org/docs/comparison-to-java.html", 200) ); var urlsAndRequests = requests.stream().collect( Collectors.groupingBy(Request::getUrl)); System.out.println(urlsAndRequests); }

In Kotlin, use the function groupBy():

class Request( val url: String, val responseCode: Int ) fun main() { //sampleStart // Kotlin val requests = listOf( Request("https://kotlinlang.org/docs/home.html", 200), Request("https://kotlinlang.org/docs/home.html", 400), Request("https://kotlinlang.org/docs/comparison-to-java.html", 200) ) println(requests.groupBy(Request::url)) //sampleEnd }

Filter elements

In Java, to filter elements from a collection, you need to use the Stream API. The Stream API has intermediate and terminal operations. filter() is an intermediate operation, which returns a stream. To receive a collection as the output, you need to use a terminal operation, like collect(). For example, to leave only those pairs whose keys end with 1 and whose values are greater than 10:

// Java public void filterEndsWith() { var numbers = Map.of("key1", 1, "key2", 2, "key3", 3, "key11", 11); var filteredNumbers = numbers.entrySet().stream() .filter(entry -> entry.getKey().endsWith("1") && entry.getValue() > 10) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)); System.out.println(filteredNumbers); }

In Kotlin, filtering is built into collections, and filter() returns the same collection type that was filtered. So, all you need to write is the filter() and its predicate:

fun main() { //sampleStart // Kotlin val numbers = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key11" to 11) val filteredNumbers = numbers.filter { (key, value) -> key.endsWith("1") && value > 10 } println(filteredNumbers) //sampleEnd }

Learn more about filtering maps here.

Filter elements by type

In Java, to filter elements by type and perform actions on them, you need to check their types with the instanceof operator and then do the type cast:

// Java public void objectIsInstance() { var numbers = new ArrayList<>(); numbers.add(null); numbers.add(1); numbers.add("two"); numbers.add(3.0); numbers.add("four"); System.out.println("All String elements in upper case:"); numbers.stream().filter(it -> it instanceof String) .forEach( it -> System.out.println(((String) it).toUpperCase())); }

In Kotlin, you just call filterIsInstance<NEEDED_TYPE>() on your collection, and the type cast is done by Smart casts:

// Kotlin fun main() { //sampleStart // Kotlin val numbers = listOf(null, 1, "two", 3.0, "four") println("All String elements in upper case:") numbers.filterIsInstance<String>().forEach { println(it.uppercase()) } //sampleEnd }

Test predicates

Some tasks require you to check whether all, none, or any elements satisfy a condition. In Java, you can do all of these checks via the Stream API functions allMatch(), noneMatch(), and anyMatch():

// Java public void testPredicates() { var numbers = List.of("one", "two", "three", "four"); System.out.println(numbers.stream().noneMatch(it -> it.endsWith("e"))); // false System.out.println(numbers.stream().anyMatch(it -> it.endsWith("e"))); // true System.out.println(numbers.stream().allMatch(it -> it.endsWith("e"))); // false }

In Kotlin, the extension functions none(), any(), and all() are available for every Iterable object:

fun main() { //sampleStart // Kotlin val numbers = listOf("one", "two", "three", "four") println(numbers.none { it.endsWith("e") }) println(numbers.any { it.endsWith("e") }) println(numbers.all { it.endsWith("e") }) //sampleEnd }

Learn more about test predicates.

Collection transformation operations

Zip elements

In Java, you can make pairs from elements with the same positions in two collections by iterating simultaneously over them:

// Java public void zip() { var colors = List.of("red", "brown"); var animals = List.of("fox", "bear", "wolf"); for (int i = 0; i < Math.min(colors.size(), animals.size()); i++) { String animal = animals.get(i); System.out.println("The " + animal.substring(0, 1).toUpperCase() + animal.substring(1) + " is " + colors.get(i)); } }

If you want to do something more complex than just printing pairs of elements into the output, you can use Records. In the example above, the record would be record AnimalDescription(String animal, String color) {}.

In Kotlin, use the zip() function to do the same thing:

fun main() { //sampleStart // Kotlin val colors = listOf("red", "brown") val animals = listOf("fox", "bear", "wolf") println(colors.zip(animals) { color, animal -> "The ${animal.replaceFirstChar { it.uppercase() }} is $color" }) //sampleEnd }

zip() returns the List of Pair objects.

Associate elements

In Java, you can use the Stream API to associate elements with characteristics:

// Java public void associate() { var numbers = List.of("one", "two", "three", "four"); var wordAndLength = numbers.stream() .collect(toMap(number -> number, String::length)); System.out.println(wordAndLength); }

In Kotlin, use the associate() function:

fun main() { //sampleStart // Kotlin val numbers = listOf("one", "two", "three", "four") println(numbers.associateWith { it.length }) //sampleEnd }

What's next?

If you have a favorite idiom, we invite you to share it by sending a pull request.

Last modified: 12 August 2022